This was the first non-fantasy game from Infocom and the first ever interactive fiction murder mystery game. It's a difficult game, for sure. I still have not completed the game, but it is worth trying out. (Infocom's Witness is definitely easier, and perhaps a better place to start.)
The game was reissued many times in the Infocom Masterpieces and Lost Treasures series. The original manual, maps, hints are all pretty widely available on various sites.
Interestingly enough there was an authorized Inform 6 rewrite whose source code is available in IFDB (though I wish there was a compiled binary file) and the original (slightly impenetrable) ZIL source code has also been posted.
If only someone would rewrite this in Inform7!
This is a great, short piece of mystery. It's the classic "locked house, dead body, 5 suspects" genre of murder mystery done in an infocom style --complete with Sgt Duffy.
For me, just coming back to the genre after many years, this was a great intro. It's got a simple map with around 10 location, not too many objects, some modest red herrings and a good *logical* story.
Not to mention that, if you do get stuck, there are built-in clues. All this makes for a good easy game for newcomers also.
The actual who/how/motives are randomized, so there's some replay value, though some of the fun is really in the first time.